Posterior vertebral joint prosthesis

ABSTRACT

The present invention relates to a posterior vertebral joint prosthesis. The left or right posterior vertebral joint prosthesis of the invention presents a smooth bearing surface and said surface presents antero-posterior curvature.

BACKGROUND OF THE INVENTION

Pain in the cervical, dorsal, and lumbar vertebrae often originates fromposterior vertebral joints or zygapophysory joints or “facet” joints.When medical therapy is no longer sufficient for easing the pain, inparticular when such joints have become very deformed by arthrosis orold trauma, it becomes desirable to replace their joint surfaces byprostheses; the purpose of such prostheses is to conserve the functionsand mobility of the vertebral column, and to avoid the joints becomingblocked (arthrodesis).

Documents FR 2 721 501 and U.S. Pat. No. 6,132,464 describe variousdevices for replacing the joint surfaces of posterior vertebral facets;each of those devices comprises a support having a convex face fittingclosely and coming into bearing contact against a portion of the concavesurface of the posterior arc of a vertebra, on at least one side of thespinous process; the support is fixed to the vertebra by a pedicularscrew, and/or to the spinous and transverse processes by encirclingmeans such as a band, a collar, or a hook; the device further comprisesa strip forming half of a joint prosthesis and having two oppositefaces: a first face known as a “joint” or as a “sliding” face which isin contact with a similar face of an adjacent vertebra with minimumfriction; said first face being covered in a biocompatible material thatslides well, such as stainless steel or titanium, in particular; and asecond face opposite from the first face of the strip, known as the“bone” face, which is porous and covered in spikes, is optionallycovered in hydroxyapatite, and is intended to bear against the remainingbone of the joint process; the strip is connected to the support by asessile base whose convex and porous posterior face bears against thebone of the vertebral arch; the means for fixing the support to thespinous and transverse processes and to the pedicle are associated withone another as a function of the anatomic conditions encountered so asto ensure stability for the assembly.

Those devices present certain drawbacks: because of the presence ofnumerous respective bearing faces on the various portions (support,strip, base) of the device on different regions of a vertebra, it isdifficult to ensure that bearing takes place with quality that isuniform and satisfactory because of the variations in shape between onevertebra and another and/or one patient and another; in addition, thepresence of the base, the anchoring support, and of the various fixingmeans contributes to increasing the volume of the prosthesis andconsequently to increasing the discomfort for the patient and the traumadue to implanting such a device.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the invention is to propose posterior vertebral jointprostheses that are improved and that remedy the drawbacks of prosthesesof this type, at least in part.

In a first aspect, the invention consists in providing a device for aleft posterior vertebral joint or for a right posterior vertebral joint,which device comprises a first prosthesis for partially replacing theanterior joint process and that includes a first sliding bearingsurface, and a second prosthesis for totally replacing the posteriorjoint process, including a second sliding bearing surface presenting atleast a portion of shape that is similar or identical to the shape of atleast a portion of said first sliding bearing surface of said firstprosthesis.

Implanting the first prosthesis for partial replacement of the anteriorjoint process merely requires prior abrasion of the joint surface (orfacet) of the anterior process, whereas implanting the second prosthesisfor total replacement of the posterior joint process is performed aftersubstantially total ablation of the posterior process.

In another aspect, the invention consists in proposing a device forreplacing posterior vertebral joint surfaces, which device comprises,for the left joint and for the right joint, a first prosthesis for theanterior joint process, the first prosthesis including a first smoothbearing surface, and a second prosthesis for the posterior jointprocess, the second prosthesis including a second smooth bearing surfaceof a shape that is complementary to the shape of said first surface ofthe first prosthesis so as to facilitate sliding on the first surfaceand so as to facilitate relative antero-posterior (or sagittal) pivotingmovement between two adjacent vertebrae fitted with these prostheses;this enables two vertebrae fitted with prostheses of the invention toconserve a high degree of mobility.

In another aspect, the invention consists in proposing posteriorvertebral joint prostheses presenting smooth sliding bearing surfacesthat are substantially V-shaped: the first prosthesis for the anteriorjoint process including a sliding bearing surface in the form of aconcave V-shape with faces preferably forming a first dihedral anglesituated in a range 90° to 160°, while the second prosthesis for theposterior joint process includes a sliding bearing surface in the formof a convex V-shape whose faces form a second dihedral angle of valueequal to or close to the 360° complement of the value of said firstdihedral angle.

Putting said first and second prostheses into mutual contact via theirrespective smooth V-shaped bearing surfaces encourages mutual guidanceof these two prostheses during their mutual sliding displacement,thereby increasing the dynamic stability of a spine in which twoadjacent vertebrae are fitted with such prostheses.

The magnitude of the open dihedral angle of the V-shaped sliding bearingsurfaces is preferably adapted to the position of the vertebra along thespine: for the lumbar-sacral L5-S1 joint, the magnitude of the angle ofthe anterior joint prosthesis is preferably close to 90° to 160° and themagnitude of the angle of the posterior joint prosthesis is respectivelyclose to 270° to 200°, whereas for the L4-L5 joint, the angle of theanterior prosthesis is preferably close to 90° to 150°, and the angle ofthe posterior prosthesis is respectively close to 270° to 210°.

In another aspect, the invention consists in providing posteriorvertebral joint prostheses presenting smooth sliding bearing surfacespresenting antero-posterior curvature: said anterior first prosthesispresents (in medial view) a rearwardly-open curved sliding surface, andsaid posterior second prosthesis presents (in medial view) aforwardly-closed curved sliding surface of curvature equal to (or closeto) the curvature of the open surface of the first prosthesis; thisfacilitates free relative antero-posterior pivoting of two adjacentvertebrae fitted with these prostheses; the antero-posterior radius ofcurvature of these sliding surfaces can be adapted to the position ofthe vertebra for which the prosthesis is intended; the magnitude of thisradius is preferably selected from a range of 18 millimeters (mm) to 28mm, more preferably 20 mm to 26 mm, and in particular close to 23 mm.

By means of this curvature, flexion-extension movements of the vertebralcolumn (antero-posterior pivoting) can be performed about a commontransverse axis for the intervertebral disk and for the joints.

In a preferred embodiment, the sliding surfaces present both a V-shapeand antero-posterior (sagittal) curvature: the anterior prosthesis has asliding surface in the form of a concave V-shape comprising arearwardly-convex medial face with a radius of curvature close to 20 mmto 26 mm, and a substantially plane lateral face; the posteriorprosthesis has a sliding surface in the form of a convex V-shapecomprising a forwardly-concave medial face with a radius of curvatureclose to 20 mm to 26 mm, and a substantially plane lateral face; inother words, in this case, said sliding surfaces extend substantiallyalong a surface of revolution generated by a curved segment turningabout a (transverse) straight line situated in a plane containing thecurved segment, and at a distance therefrom which is equal to saidradius of curvature; as a result, the sliding surfaces of the left andright prostheses are substantially in the shape of portions of a kind ofcircularly toroidal groove.

In another aspect, the invention consists in providing an anteriorprosthesis for a posterior vertebral joint, the prosthesis comprising asmooth sliding bearing surface and a bone bearing surface, together withan anchor structure for anchoring the prosthesis in the joint processand projecting from said bone bearing surface; the invention alsoprovides a posterior prosthesis for a posterior vertebral joint, theprosthesis comprising a sliding bearing surface and a bone bearingsurface, together with an anchor structure for anchoring the prosthesisin the vertebral isthmus and/or in the vertebral lamina, which structureprojects from said bone bearing face.

The invention makes it possible to obtain prostheses that are welladapted to human anatomy, that are compact, stable, and not verytraumatizing.

Preferably, each of the anchor structures comprises an elongate anchormember—such as a stud or a rod—extending along an axis which isgenerally inclined relative to the normal to the bone bearing surface;said anchor member is preferably substantially cylindrical in shapeabout said axis, and is threaded or provided with grooves or projectionson its outer face in order to increase bone contact area.

Specifically, for an anterior prosthesis, the anchoring that resultsfrom this member can be improved by the presence of a rib projectingalong a portion of the periphery of the bone bearing surface, therebyincreasing contact area with the bone.

Specifically, for a posterior prosthesis, anchoring can be improved bythe presence of two elongate anchor members—such as rods orstuds—extending along two substantially orthogonal axes.

Said elongate anchor members may form a single part together with theprosthesis body that includes the bone bearing face and the slidingbearing face; alternatively, or in addition, a distinct anchor membermay be used, in particular in the form of a screw passing through anorifice provided in the prosthesis body.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and characteristics of the invention will be understoodon reading the following description which refers to the accompanyingdrawings which show preferred embodiments of the invention that are ofno limiting character.

FIG. 1 is a diagrammatic dorsal view of the top portion of the sacrumand of lumbar vertebrae L4 and L5, with four left posterior jointprostheses being shown thereon replacing two superposed left posteriorjoints.

FIG. 2 is a diagram in cranial view of a left anterior joint prosthesisand a left posterior joint prosthesis replacing a posterior jointbetween two successive vertebrae of the spine.

FIG. 3 is a diagrammatic perspective view showing the shape of thesliding bearing surfaces of prostheses of the invention.

FIG. 4 is a diagrammatic side view of vertebrae fitted with jointprostheses and how they move in mutual pivoting about a transverse axis.

FIGS. 5 to 8 show an embodiment of an anterior joint prosthesis adaptedto the lumbar-sacral joint L5-S1, and FIGS. 9 to 11 show a posteriorjoint prosthesis complementary to the prosthesis of FIGS. 5 to 8.

Like FIGS. 5 to 8, FIGS. 12 to 15 show embodiments of an anterior jointprosthesis adapted for the posterior joint between lumbar vertebrae L4and L5, and FIGS. 16 to 18 show a posterior joint prosthesiscomplementary to the prosthesis of FIGS. 12 to 15.

FIGS. 5, 9, 12, and 16 are cranial views of the prostheses; FIGS. 6, 10,13, and 17 are medial views of the prostheses; FIGS. 17 and 14 aresagittal views; FIGS. 8 and 15 are horizontal views; and FIGS. 11 and 18are lateral views.

FIGS. 7, 8, 14, and 15 are section views.

MORE DETAILED DESCRIPTION

Although the invention is described below in association with the lumbarportion of the spine, the invention is applicable to the posteriorjoints of dorsal and cervical vertebrae as well.

With reference to FIGS. 1 and 2 in particular, a left or right posteriorvertebral joint is replaced by using an anterior prosthesis 1 referredto as a “first” prosthesis, and a posterior prosthesis 2 referred to asa “second” prosthesis. The anterior prosthesis 1 is fixed on a vertebrasuch as L5 or to the sacrum S1, while the posterior prosthesis 2 isfixed to the vertebra situated immediately thereabove, such as L4 or L5,respectively.

With reference to FIGS. 5 to 18 in particular, each of the prostheses 1and 2 comprises a body 6, 7 and a cylindrical anchor rod 3, 5 integratedin the body and elongate along an axis 8, 9.

With reference to FIGS. 5 to 8 and 12 to 15, the body 7 of the anteriorprosthesis 1 is defined to a large extent by two opposite surfaces: asurface 10 enabling the prosthesis to bear against the stump of theanterior joint process, and a surface 11 for sliding contact with theposterior prosthesis (reference 2); as shown in particular in thecranial views (FIGS. 5 and 12), the surface 11 has two faces 11 a and 11b forming a kind of V-shape with a rearwardly-open dihedral angle 12 ofabout 140° for the L5-S1 prosthesis of FIG. 5 and close to 120° for theL4-L5 prosthesis of FIG. 12; in addition, these faces 11 a, 11 b extendalong an intermediate surface 15 a, 15 b which is circularly cylindricalabout a transverse axis 13 having a radius of curvature 14 (see FIGS. 3and 4) that is close to 23 mm; the medial portion 11 a of the slidingsurface 11 is convex, while the lateral portion 11 b of the surface 11is plane.

In complementary manner, each posterior prosthesis 2, FIGS. 9 to 11 and16 to 18, presents a surface 16 for bearing against the bone and asurface 17 for bearing in sliding manner on the surface 11 of thecorresponding prosthesis 1; the surface 17 comprises a face 17 a and aface 17 b that form a convex V-shape (FIGS. 9 and 16) with an opendihedral angle 18 of about 220° (FIG. 9) or about 240° (FIG. 16); themedial portion 17 a of the sliding surface 17 is concave while thelateral portion 17 b is substantially plane.

The concave side of the V-shape 11 a, 11 b of the anterior prosthesisfaces rearwards, whereas the convex side of the V-shape 17 a, 17 b ofthe posterior prosthesis faces forwards.

The L5-S1 posterior prosthesis (FIG. 10) includes an orifice 19specially designed to receive an additional anchor screw (referenced 4,FIGS. 1 and 2) for anchoring the prosthesis in the lamina 20 of thevertebral arch.

Retention of the prostheses 1 and 2 is reinforced by the presence ofstabilizing ribs 21 (see FIGS. 5, 7, and 8, in particular), 22, 23, and24 (see FIGS. 10 and 17) which project from the periphery of thebone-bearing surface 10, 16; with the posterior prosthesis 2, the ribs22 to 24 and the surface 16 define a groove or gully.

Preparation for implanting the prostheses requires a posterior approachexposing the posterior joint(s) to be replaced. At least partialexcision (ablation) of the joint processes is necessary. On the anteriorfacet (or joint process) (or on the superior joint process of theunderlying vertebra) the joint surface is merely abraded so as to reducethe total volume of the facet and provide a sagittal anchor hole in itslower portion. On the posterior facet (or joint process) (or on thelower joint process of the underlying vertebra) sub-total ablation isperformed with oblique section towards the vertebral lamina andperpendicular section over the vertebral isthmus, and two anchor holes,a sagittal hole on the axis of the isthmus towards the vertebralpedicle, and a transverse hole in the thickness of the vertebral laminaare hollowed out from the spinal process.

The anterior prosthesis 1 bears against the prepared stumpof-the-anterior facet with its cylindrical anchoring stud penetrating inthe anchor hole. The prosthesis presents a smooth joint surface 11, 11a, 11 b on its rearwardly-facing face of a shape that presents arearwardly-open V-shaped section (or a section like a pulley wheelgroove), with a narrow front medial sector and a broad and obliquelateral sector. In horizontal section, the medial sector is a frontalsector and the lateral sector presents obliqueness lying in the range50° to 20° going from the sacrum to the top of the lumbar column. Insagittal section, the medial and lateral sectors 11 a and 11 b of theprosthesis have rearwardly-directed convex sides corresponding to thearc of a circle whose center corresponds to the common pivot centerbetween the two posterior joints and the intervertebral disk for a givenmoving segment of the spine (the radius of this circle is 23 mm onaverage).

The posterior prosthesis 2 presents a first integral stud 3 provided forengaging in the anchor hole in the isthmus, and a second stud 4 which isseparate from the body of the prosthesis and which is screwed foranchoring to the lamina. The forwardly-directed sliding joint surface17, 17 a, 17 b bears intimately against the joint surface 11, 11 a, 11 bof the anterior prosthesis 1.

The prostheses 1 and 2 may be made of metal or of plastics material orof any other material that is biologically compatible. The retention ofthe prostheses can be reinforced by means of plastic cement.

1. A left or right posterior vertebral joint prosthesis presenting asmooth sliding bearing surface, wherein said surface presentsantero-posterior curvature, and pivoting guide means suitable forco-operating with a complementary bearing surface of said smooth slidingbearing surface.
 2. A prosthesis according to claim 1, in which theguide means comprise two faces or face portions forming part of saidsmooth sliding bearing surface and substantially forming a V-shape.
 3. Aprosthesis according to claim 2, in which said faces or face portionsextend along a circularly cylindrical surface presenting saidantero-posterior curvature.
 4. A prosthesis according to claim 1, inwhich the radius of curvature of said smooth sliding bearing surfacelies in the range 18 mm to 28 mm.
 5. A prosthesis according to claim 1,further comprising a bone bearing surface and an anchor structure, andin which the anchor structure projects from the bone bearing surface. 6.A prosthesis, according to claim 5, in which the anchor structureincludes an anchor member that is elongate an axis that is inclinedrelative to the normal of the bone bearing surface.
 7. A prosthesisaccording to claim 6, in which the anchor member is substantiallycylindrical in shape.
 8. A prosthesis according to claim 6, in whichsaid anchor member is provided on its outside surface with grooves orprojections.
 9. A prosthesis according to claim 5, in which at least aportion of the anchor structure is integral with the body of theprosthesis.
 10. A prosthesis according to claim 1, for replacing theanterior joint process, in which at least a portion of said smoothbearing surface is convex, rearwardly open, and curved with a radiuslying in the range 18 mm to 28 mm.
 11. A prosthesis according to claim10, for replacing the anterior joint process, in which the faces of theV-shape form a dihedral angle lying in the range 90° to 160°.
 12. Aprosthesis according to claim 1, for replacing the posterior jointprocess, in which at least a portion of said smooth bearing surface isconcave, forwardly closed, and curved with a radius lying in the range18 mm to 28 mm.
 13. A prosthesis according to claim 12, for replacingthe posterior joint process, in which the faces of the V-shape form adihedral angle lying in the range 200° to 270°.
 14. A device forreplacing a left or right posterior vertebral joint, the devicecomprising: a prosthesis for partially replacing the anterior jointprocess, referred to as the “first” prosthesis, which prosthesis ispreferably in accordance with claim 1, and has a first smooth slidingbearing surface in the form of a pulley groove; and a prosthesis forreplacing the posterior joint process, referred to as the “second”prosthesis, which is preferably in accordance with claim 1, and whichincludes a second smooth sliding bearing surface, of which at least aportion is similar or complementary in shape to at least a portion ofsaid first smooth sliding bearing surface of said first prosthesis, soas to facilitate relative sagittal pivoting movement between twoadjacent vertebrae fitted with these prostheses.